The invention relates to a device and to a method for controlling the transport of a tape within a tape transport system based on a primary velocity measurement of the tape and a secondary velocity measurement of the tape.
In modern tape storage systems, the capacity and performance of the tape storage systems have increased considerably. To achieve higher cartridge or tape capacities and improved performance, however, further advances in several areas are necessary. Increases in linear and track densities on the tape may be required in order to achieve higher storage capacities. However, increase in linear densities may lead to a decrease of the distance between adjacent bit cells, which in turn may lead to an increase of inter-symbol interferences. Increase in track densities may lead to narrower individual track widths and narrower write and/or read heads which may require a very precise control of the tape transport system and track-follow control of the tape head. Thus, reliable and precise tape transport is of importance to guarantee read-channel performance on all parallel data channels during tape operation.
To achieve a reliable and precise tape transport, tight control of tension and potentially of tape dimensional stability (TDS) variations may become necessary. This may lead to the use of thinner tape material, which in turn may enable an increase in volumetric density. Usually, the performance of the tape transport servomechanism and the quality of readback signals in data channels are affected by variations in the tape velocity and tension. For instance, during operation in cruise velocity mode, variations of tape velocity and tension around the nominal value, also called once-around, may be induced by reel eccentricities. This problem may become critical when the reel rotation frequencies are near the resonance frequency determined by the tape path.
Conventionally, for the tape transport operation, a dual servo channel provides estimates of the tape velocity, tape longitudinal position, and head lateral position, which are derived from servo signals that are read by servo readers in the head module. In current tape transport control systems, the tape velocity measured at the head using pre-formatted servo information, the so called primary velocity, is the only velocity information that is used for a velocity control during cruise mode. Hall sensors can be used to obtain secondary tape velocity information from the individual reels, which information is typically used to achieve proper tape transport operation in the absence of valid parameter estimates from the servo channel that provide the primary velocity measurement. This may be the case for instance during acceleration or deceleration of the tape. Such a system is for example disclosed in U.S. Pat. No. 6,754,026 B1.